The invention relates to solid state apparatus for the generation of AC power in general, and more particularly to a transistorized self-oscillatory static inverter utilizing feedback control for the synthesis of the outputted sine wave.
For many applications, in particular for an uninterruptible power supply (UPS) such as can be used for instance on a spacecraft, or in a computer facility, there is a need for an inverter power stage of low weight and cost, which is simple, quiet operating and rugged, that can deliver sufficient power with a good sine wave at an easily selectable frequency and an easily selectable output voltage. Another requirement is that the solid state devices be used to a full rating and with the smallest number of components in order to maximize the power output while minimizing cost. In this respect, the present invention provides for an improved transistorized static inverter power stage.
The present invention makes use of waveform synthesis with time-optimal-response by feedback control of the static switches of an inverter. This technique is essentially a self-oscillatory static inverter approach, known for its excellent transient response and active filtering which capabilities attenuate greatly any load or line changes.
The self-oscillatory static inverter technique consists in comparing, at the driving input of the static inverter, the outputted sine wave at the fundamental frequency with a reference sine wave of the same frequency. "Bang-bang" control is introduced via the comparator by causing switching of the system with the feedback error fluctuating alternately between an upper and a lower limit, or hysteresis limits, established above and below the reference voltage. Moreover, the lags of the system are anticipated in a second feedback control loop adding by differentiation a rate error to the error voltage of the first loop, which second feedback control loop is rendered insensitive to the fundamental frequency output voltage.
Due to the anticipatory function, switching events occur at unique times before error voltage alone reaches either of the said hysteresis limits. The effect is that potentially large overshoots, or undershoots, are easily contained, thereby adding stability and insuring proper control within the hysteresis limits. Bang-bang control according to the present invention also uses two more refinements in the form of additional control loops. A three-loop control results when the master comparator is provided with a derived reference voltage target which corrects for the fact that the average output voltage is likely not to lie half-way between the two hysteresis limits. A four-loop control provides additionally, for synchronous operation at a switching rate made an integral multiple of the output fundamental frequency. Self-oscillation normally implies asynchronous switching rate which results in apparent voltage modulation and audible noise as additional perturbing factors. The form of modulation produced through the feedback control system when so synchronized is identified as synchronous pulse frequency modulation (S.P.F.M.).
The bang-bang technique has been disclosed by applicant in U.S. Pat. Nos. 3,636,430 and 3,648,150.
The bang-bang feedback control method of waveform generation has been performed in the past with a four-transistor bridge like in the aforementioned Kernick patents. A variation with only two transistors has also been designed which involves a center-tap power stage and an Andre choke, as explained on page 299 in relation to FIG. 1 in "Static Inverter With Synchronous Output Waveform Synthesized By Time-Optimal-Response Feedback" by A. Kernick, D. L. Stechschulte and D. W. Shireman in IEEE Vol. IECI-24, No. 4, November 1977.
In the bang-bang mode, the static switching devices of the inverter bridge are alternately controlled for conduction, one under positive polarity and the other under negative polarity, so that the ON/OFF status of the devices be complementary, e.g., each device provides the complementary chopped sine wave modulation along the time axis for both half-cycles of the fundamental. The power transformer has its primary in series with a filter choke and this series network is across the opposite poles of the bridge. The secondary of the transformer outputs the fundamental sine wave from which feedback control is derived. Besides, as explained in the article, an output capacitor is mounted across the higher voltage winding, usually the secondary, and current in this capacitor is sensed so as to obtain anticipation.
The article of Kernick, Stechschulte and Shireman also describes a variation from the "bang-bang" control technique which is labelled by the authors as the time-optimal-response "bang-hang" approach. By "bang-hang" is meant that the static switches in the bridge are now controlled for three possible states, rather than the mere two opposite states ON/OFF defined in the bang-bang mode alternately. An additional state is provided corresponding to zero polarity drive on the series network, whereby both transistors connected to either the upper or the lower power rail are simultaneously turned ON, thereby to short-circuit the series combination of the filter choke and the primary of the transformer in the diagonal of the bridge. During such a no-polarity "hang" condition, the energy stored in the choke is either furnished to or augmented by the load in either a transient decay or a transient build-up used in the waveform synthesis of a segment of the time-optimally-controlled output. Choke-stored energy so used in an interchange with the load constitutes an active filter mode of operation.
It is also known from U.S. Pat. No. 3,614,590 of A. Kernick to synthesize a sine wave outputted at a fundamental frequency through a bridge of four transistors arranged in two half-bridges, or poles, across DC terminals. Two transistors of one pole are controlled for alternate conduction at said fundamental frequency, the two other transistors of the other pole are controlled in a pulse-width-modulation mode by feedforward control through the generation of a picket-width modulation type of programmed waveform applied to the transistors in accordance to a predetermined logic pulse generator controlled by a base oscillator of a frequency substantially higher than said fundamental frequency. With this approach, however, four transistors have been used of high rating and cost to achieve maximum power output. Moreover, feedforward is not as desirable as feedback control which allows response in the output to follow closely load and line changes, thereby improving the transient quality of the generated waveform.
The present invention provides an improved two-transistor inverter stage for the generation of AC power. The circuit according to the invention combines the use of transistors at their full rating and of thyristor devices. In place of SCR devices, it is also possible to use other thyristor types of high power, solid state devices, for instance GTO devices. The circuit is simple, rugged, and it permits to generate synchronously an excellent sine wave which can be easily controlled for desired output frequency and output voltage.